Blind fasteners, or rivets, as well known in the art, are utilized in fastening components in which only one side of a workpiece is accessible. U.S. Pat. No. 3,148,578 to Gapp describes a fastener including a tubular fastener body or sleeve with an elongated stem extending through the sleeve. One end of the stem is adapted to be gripped and pulled by a pulling tool while the opposite end has an upsetting head. Adjacent a tail end of the sleeve is a shear ring integrally formed with the stem.
In operation, the Gapp assembly is inserted through aligned holes in workpieces to be secured together. A tool is used to pull the upsetting head of the stem axially into the tail of the sleeve, while a reactionary force is applied against a head of the sleeve. During this first part of the pulling operation, the shear ring engages the sleeve tail which causes radial expansion of the tail and bulbing on the blind side of the workpieces. The shear ring is constructed to shear from the stem at a predetermined tension in the stem, thereby limiting the amount of clamping compression on the workpiece to a predetermined value. Continued pulling causes a lock ring groove in the stem to reach the sleeve head where the lock ring is deformed into the groove. Continued pulling increases the load, causing the stem extending beyond the accessible face of the workpiece to break away.
U.S. Pat. No. 4,012,984 --Matuschek--discloses a design similar to that in the Gapp patent with the addition of a lock ring surrounding the stem and moving with the stem into the sleeve head area.
In other patents such as U.S. Pat. No. 5,052,870--Pratt, et al., a separate shear ring anchored in a groove in the stem is employed instead of an integral shear ring as in the Gapp and Matuschek patents. Also, to ensure that the workpiece plates are drawn together before the sleeve is expanded to fill the hole, the shear ring is provided with an axially tapered forward surface which mates with an inwardly tapered surface on the tail of the sleeve. This facilitates the bulbing of the sleeve tail and ensures good workpiece clamp-up. While there are a number of advantages to the arrangement described in U.S. Pat. No. 5,052,870, the integral shear ring fastener of the Gapp and Matuschek patents also has some advantages and has been used in very successful fasteners.
One disadvantage of the integral shear ring fastener is that the shear ring can cause axial compression, and hence radial expansion of the sleeve before the workpieces have been fully clamped together. The forward end of an integral shear ring may be tapered to mate with a tapered sleeve end as in U.S. Pat. No. 5,052,870 and thus ensure good workpiece clamp-up. This, however, presents some manufacturing limitations and difficulties in that it is necessary that the axial width of the shear ring be precisely controlled so that it will shear with the desired load. If the forward end of the shear ring is tapered with its small diameter towards the sleeve head end and its larger diameter away from the sleeve head, it is necessary to taper the back face of the shear ring in a similar manner in order to control the axial width of the shear ring. This undercut type machining operation is somewhat difficult and expensive. It also largely precludes the use of stamping or coining operations to form the integral shear ring. A need exists for an improved integral shear ring type fastener that will provide the desired workpiece clamp up but will, nevertheless, have simplified manufacturing fabrication techniques.